wext is short for "Wireless EXTension", and all it means is that it should use the default wireless card on your system and you don't have to change anything anywhere.
This is what I would like you to do:
1 - open a terminal and run:
tail -f /var/log/messages
That will read kernel messages, so leave that running.
2 - Open a second terminal and run only the wpa_supplicant command:
wpa_supplicant -B Dwext -iwlan0 -c/etc/wpa_supplicant.conf
Then post all the messages that appear the both the terminals here so we can take a look at them.
I'm sorry but I don't speak french (I do know a few of the basics, but not enough to engage in a full conversation